There are currently a wide array of surgical devices for spinal surgery. One group of these devices includes a fixation device including a screw or hook anchor for securing the device to a vertebrae, such as the pedicle portion of the vertebra. Many of these surgical devices are spinal rod fixation devices and include a coupling portion for receiving or otherwise securing a spinal rod or other elongated member relative to the anchor. Commonly, the coupling portion or yoke is a distinct member from the anchor so that they can take on a plurality of different orientations relative to each other.
For example, for a screw anchor member, a threaded shank of the screw anchor is driven into the pedicle portion at a particular angle. At times, the orientation and position of the threaded shank with the vertebra is dictated by the surface geometry or other characteristics particular to the patient and/or to the individual vertebrae. However, the spinal rod that is to be coupled with and fixed relative to the screw anchor has a configuration and position that is dictated more by the goals of the medical procedure. To allow the yoke to receive the spinal rod, while also permitting the desired positioning of the spinal rod, the yoke member may assume a number of orientations relative to the screw anchor so that their respective axes are other than in alignment with each other.
During installation, this polyaxial feature presents a number of issues. The coupling member is not secured relative to the screw anchor during insertion. Thus, the coupling member tends to pivot by its own weight as the screw is being driven into the vertebral bone. This can be problematic as the coupling member may interfere with the rotational driving action, such as by binding with the driver and screw anchor, and this interference may inadvertently alter the position and orientation in which the shank is driven.
It is desired to minimize the resection required to implant the fixation device and its associated coupling member. In the event that the coupling member tilts or pivots from alignment with the axis of the screw anchor, the coupling member may contact tissues surrounding the implantation site for the fixation device. As the screw member turns with the coupling member tilted to one side, the coupling member may sweep around the implantation site potentially causing damage to surrounding tissues. As is apparent, the polyaxial movement provided between the screw anchor and yoke coupling member can make the driving operation for implanting the screw into the vertebral bone more difficult than is desired.
In general, it is preferred to minimize the incision and resection performed on a patient for most operations. For instance, arthroscopy and endoscopes were developed to permit inspection and surgery for interior anatomical portions while avoiding significant incisions through healthy and non-damage tissues. As such, it is also desired to minimize the size of the fixation device including the coupling member thereof during implantation.
There are numerous tools devised for implanting the various designs of fixation devices and coupling devices. A prior art device is further known for seating a screw in the pedicle portion of a vertebrae, the screw having an associated coupling device, wherein the prior art device generally stabilizes the coupling device relative to the screw during driving. More specifically, U.S. Pat. No. 6,858,030, to Martin, et al., discloses a driver for implanting a screw of a pedicle screw assembly that includes a polyaxial coupling element. The driver has threads spaced up along its shaft from the lower driver end for being received in internal threads formed along the inner surface of the walls of the coupling element. Internally threading these walls necessitates that the walls be of sufficient thickness for forming threads therein, undesirably increasing the size or width of the coupling element, as well as the height of these walls along which the threads are formed. Seating of the drive prongs in the corresponding recesses of the screw head is difficult due to the need to positively thread the driver into the coupling element. Further, after the screw has been fully implanted in the pedicle of the vertebra, releasing the driver requires it be turned to back the shaft threads out from the coupling element threads, potentially loosening the implanted screw.
In fact, the driver and screw head of the '030 patent require a linear engagement, while the shaft is rotated into the coupling element. To achieve this, the coupling element must be rotated relative to both the driver and the screw so that they are drawn together (or forced away) by the coupling element. Disconnection of the shaft and coupling element would require a surgeon's fingers, or another device, reaching into the implant site and rotating the coupling element a plurality of revolutions.
Accordingly, there is a need for a device for an optimized apparatus and method for implanting screw and anchor members of spinal rod fixation devices having a polyaxial coupling member in terms of the ease in which the screw anchor is driven into the vertebral bone. In addition, an apparatus and method for implanting screw anchors is desired that keeps the size of the device, and in particular the coupling member, to a minimum.